From nobody Thu Apr 2 19:00:22 2026 Received: from mail-wr1-f73.google.com (mail-wr1-f73.google.com [209.85.221.73]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id C35B524337B for ; Mon, 16 Feb 2026 14:50:40 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.221.73 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771253442; cv=none; b=bt39oFAj+NUjnpZsmtyTKyPEkCvDJ57hYrdWk4++NXKUP799PE2voT7BCSEBzVyax59EEfc2yGi3nSp6L/q3+TW7shtO9zH9IsZ/JOeZzMWG5PEwMY9gTbsG6wwTHeZOO7sTY9StLlREukCNzQ1oUGygt2OaKS7Ng7cpabqaTnA= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771253442; c=relaxed/simple; bh=Ip1FGe9THDnU9YMANqAopIxPtcAKpyNKkQY8BLBr/bU=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=Uav1GfIQr8DxOhoRxvcwLtdS4xchVAimRmr3Vga9JYm+RoywMG0B+dLejy0+4O1OxKynK1u7HDcad+qyuC2eUN9o+HhMh23J/FvvNAb8iGTYd6k72ozdD7D3Oqfn60ACdrTI6n09kNwSae5tFu9iIqCnQiX+bbIeplz/niuRjgY= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--elver.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=BHnXSCp7; arc=none smtp.client-ip=209.85.221.73 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=flex--elver.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="BHnXSCp7" Received: by mail-wr1-f73.google.com with SMTP id ffacd0b85a97d-4368060a5e5so3677143f8f.3 for ; Mon, 16 Feb 2026 06:50:40 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1771253439; x=1771858239; darn=vger.kernel.org; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=zYdVr7BD//ML3cPspm8e51p1r/jn9khxfOY35saBUOQ=; b=BHnXSCp7p8Pl5Cddau5YTfFEDzPNX/SInMx3RtOh/QCiUZsiL+2vHYMT2N2GAYeInl x0nNOcPsjWCgf2ksxmeLvKNlCIoIDZ0CQmLPvhMIXzABBzdfr+Ji13e88pvGDyLJeWW0 2eBug3iVlTJyE7y8igSLoIxmqZuQG3vKvksTKljmQ9s3oIVMIYrSwKgLbwnBmRhZRLc8 3ff/EjNQ5B5I9xR3Y8oqJRD4LJZY/MnbwpR14asiJDatOU6YEyYr14Q5U4x2kvEwiMzS h1kqShfVqBnQ0we6/ohXXVPsdqSiLu49io2Dq7zb0MJ07Jdk8cUP41DK0NGGScb+Pr2R w2MQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1771253439; x=1771858239; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=zYdVr7BD//ML3cPspm8e51p1r/jn9khxfOY35saBUOQ=; b=PgyGKxYpTu620YTN+JhCHKEQOidkDzR9+H1zk8/ASMPlDwz8SOoIvRoxdPs8I9LA9P bC92BfG6YlikI/BWA7r2e7ShBpQEG0jwojCgvABfKjwAAzWT72GjFKE1lrJgz2nAL0/u FDPgb4BmVJzSdaDftr9SW1O2b9BFVD8TNk+2y+hDebPASHPD3WOGVZnfaBanDcJp7RTO utnri7mHWZlSwEhpTBtGiYDscjj3bXdWFFYZcx8cmmERIopgcqValKX/P9aZ4fYemgT2 n8htMlzTgCne8H4gwsdLhQjCMyRyECvb5kwHckrURuBZmgaci7m6f7/ZT7IpnXJzD0yd o4/w== X-Forwarded-Encrypted: i=1; AJvYcCVwgDrr1P6e5OPdoUhs/nSFF4P77obTrrF8cCuGgWkQr1AqAGgJmOACq/LbSdDMO6eEnMVCksADoxeWMh4=@vger.kernel.org X-Gm-Message-State: AOJu0YwIDv4zYLFXasjMTIkazi/8vcbHOCFEwK4bH6KKhhBtZvfaTe/8 xjErUNxCoC1dUMxs9ihZ/8evK8M7kJOTtZwJpLXjk6zz6v8mANd8rGzDjRTm2vulwzSPmDAnHcD GPA== X-Received: from wrqa11.prod.google.com ([2002:adf:f7cb:0:b0:437:72d9:7316]) (user=elver job=prod-delivery.src-stubby-dispatcher) by 2002:a05:6000:613:b0:435:a0ca:bdca with SMTP id ffacd0b85a97d-4379dbae796mr14728899f8f.57.1771253438836; Mon, 16 Feb 2026 06:50:38 -0800 (PST) Date: Mon, 16 Feb 2026 15:16:23 +0100 In-Reply-To: <20260216142436.2207937-2-elver@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20260216142436.2207937-2-elver@google.com> X-Mailer: git-send-email 2.53.0.335.g19a08e0c02-goog Message-ID: <20260216142436.2207937-4-elver@google.com> Subject: [PATCH v4 2/2] arm64, compiler-context-analysis: Permit alias analysis through __READ_ONCE() with CONFIG_LTO=y From: Marco Elver To: elver@google.com, Peter Zijlstra , Will Deacon Cc: Ingo Molnar , Thomas Gleixner , Boqun Feng , Waiman Long , Bart Van Assche , llvm@lists.linux.dev, David Laight , Al Viro , Catalin Marinas , Nathan Chancellor , Arnd Bergmann , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, kernel test robot , Boqun Feng Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" When enabling Clang's Context Analysis (aka. Thread Safety Analysis) on kernel/futex/core.o (see Peter's changes at [1]), in arm64 LTO builds we could see: | kernel/futex/core.c:982:1: warning: spinlock 'atomic ? __u.__val : q->loc= k_ptr' is still held at the end of function [-Wthread-safety-analysis] | 982 | } | | ^ | kernel/futex/core.c:976:2: note: spinlock acquired here | 976 | spin_lock(lock_ptr); | | ^ | kernel/futex/core.c:982:1: warning: expecting spinlock 'q->lock_ptr' to b= e held at the end of function [-Wthread-safety-analysis] | 982 | } | | ^ | kernel/futex/core.c:966:6: note: spinlock acquired here | 966 | void futex_q_lockptr_lock(struct futex_q *q) | | ^ | 2 warnings generated. Where we have: extern void futex_q_lockptr_lock(struct futex_q *q) __acquires(q->lock_ptr= ); .. void futex_q_lockptr_lock(struct futex_q *q) { spinlock_t *lock_ptr; /* * See futex_unqueue() why lock_ptr can change. */ guard(rcu)(); retry: >> lock_ptr =3D READ_ONCE(q->lock_ptr); spin_lock(lock_ptr); ... } At the time of the above report (prior to removal of the 'atomic' flag), Clang Thread Safety Analysis's alias analysis resolved 'lock_ptr' to 'atomic ? __u.__val : q->lock_ptr' (now just '__u.__val'), and used this as the identity of the context lock given it cannot "see through" the inline assembly; however, we want 'q->lock_ptr' as the canonical context lock. While for code generation the compiler simplified to '__u.__val' for pointers (8 byte case -> 'atomic' was set), TSA's analysis (a) happens much earlier on the AST, and (b) would be the wrong deduction. Now that we've gotten rid of the 'atomic' ternary comparison, we can return '__u.__val' through a pointer that we initialize with '&x', but then update via a pointer-to-pointer. When READ_ONCE()'ing a context lock pointer, TSA's alias analysis does not invalidate the initial alias when updated through the pointer-to-pointer, and we make it effectively "see through" the __READ_ONCE(). Code generation is unchanged. Link: https://lkml.kernel.org/r/20260121110704.221498346@infradead.org [1] Reported-by: kernel test robot Closes: https://lore.kernel.org/oe-kbuild-all/202601221040.TeM0ihff-lkp@int= el.com/ Cc: Peter Zijlstra Tested-by: Boqun Feng Reviewed-by: David Laight Signed-off-by: Marco Elver --- v3: * Use 'typeof(*__ret)'. * Commit message. v2: * Rebase. --- arch/arm64/include/asm/rwonce.h | 10 +++++++--- 1 file changed, 7 insertions(+), 3 deletions(-) diff --git a/arch/arm64/include/asm/rwonce.h b/arch/arm64/include/asm/rwonc= e.h index 9fd24cef3376..0f3a01d30f66 100644 --- a/arch/arm64/include/asm/rwonce.h +++ b/arch/arm64/include/asm/rwonce.h @@ -42,8 +42,12 @@ */ #define __READ_ONCE(x) \ ({ \ - typeof(&(x)) __x =3D &(x); \ - union { __rwonce_typeof_unqual(*__x) __val; char __c[1]; } __u; \ + auto __x =3D &(x); \ + auto __ret =3D (__rwonce_typeof_unqual(*__x) *)__x; \ + /* Hides alias reassignment from Clang's -Wthread-safety. */ \ + auto __retp =3D &__ret; \ + union { typeof(*__ret) __val; char __c[1]; } __u; \ + *__retp =3D &__u.__val; \ switch (sizeof(x)) { \ case 1: \ asm volatile(__LOAD_RCPC(b, %w0, %1) \ @@ -68,7 +72,7 @@ default: \ __u.__val =3D *(volatile typeof(*__x) *)__x; \ } \ - __u.__val; \ + *__ret; \ }) =20 #endif /* !BUILD_VDSO */ --=20 2.53.0.335.g19a08e0c02-goog